Secretions

Question 1

Mucous vs. Mucus vs. Mucin

Answer 1

o Mucous glands secrete mucus which is bicarbonate rich to buffer and lubricate the GI tract; mucin is glycoprotein that helps form the viscosity of mucus

Question 2

Thirst - consequences of dehydration and hormonal influences

Answer 2

o Consequences of dehydration – hypovolemia, hypertonicity stimulates thirst and compensation (ADH release)
o Rise in plasma osmolarity (concentration of “stuff”) is detected by receptors in hypothalamus signals thirst to
o Hormonal influences: ADH, Ang II, aldosterone (elevations in these hormones promote sodium and/or water conservation during dehydration)

Question 3

Hunger and Satiety - hormones that regulate feeding

Answer 3

 Ghrelin secreted into blood during fasting state and stimulates hypothalamic neuropeptide Y to stimulate hunger
 Leptin - produced in adipose cells and suppresses appetite by decreasing neuropeptide Y
 During feeding, peptide YY and glucagon-like peptide-1 (GLP-1) from lower small intestine L cells are secreted into the blood suppressing neuropeptide Y and decreasing appetite

Question 4

Oral Secretions

Answer 4

 Salivary glands secrete saliva that lubricates food and starts digestion of starches (amylase) and lipids (via lingual lipase secreted by Von Ebner’s glands in tongue)
 R protein secreted to protect vitamin B12 from digestion by proteases

Question 5

Salivary Glands

Answer 5

 Parotid glands secrete serous saliva
 Submaxillary glands secrete mixed serous/mucous saliva
 Sublingual glands secrete mucous

Question 6

Saliva Formation

Answer 6

 1.5 L/day is secrete; regulated by parasympathetics (glossopharyngeal and facial nerves – NOT VAGUS)
 Acinar cells responsible for primary secretion of saliva; increase in flow rate increases the osmolarity due to decrease in reabsorption of electrolytes
 Myoepithelial cells – innervated by alpha-adrenergic fibers and can contract
 Striated ducts – secretion and absorption of electrolytes as the primary secretion flow through the ducts changes composition of saliva

Question 7

Final Tonicity of Saliva

Answer 7

o Final tonicity of saliva is dependent on flow through the ducts; will ALWAYS be HYPOtonic to plasma; HCO3- is primarily absorbed in the ducts
 Increase in flow rate increases osmolality due to decrease in reabsorption of electrolytes

Question 8

Autonomic Control of Oral Secretions

Answer 8

 Parasympathetics via Ach, VIP (can NOT salivate without) –via glossopharyngeal and facial nerve (NOT VAGUS)
 Sympathetic via beta and alpha adrenergics (causes constriction of myoepithelial cells to release its contents)
 Central nervous system via afferents and efferents to salivary nucleus

Question 9

Hormone Control of Oral secretions

Answer 9

 ADH increases water absorption, concentrating saliva

 Aldosterone increases K secretion into saliva and Na out of saliva

Question 10

Factors Increasing Oral Secretion Flow

Answer 10

parasympathetics, CNS (cephalic phase), nausea, esophageal distension, chewy/flavorful food, dry, acidic alkaline foods, meats, sweets, bitter foods

Question 11

Factors Decreasing Oral Secretion Flow

Answer 11

– sympathetic, ADH/Aldo hormones, sleep, dehydration, drugs, aging

Question 12

Functions of Saliva

Answer 12

 Coagulation – coagulation factors and platelet-activating factor
 Oral hygiene – buffers, cleansing, importance of flow, antimicrobial activity
 Lubrication – functions of mucin, water
 Digestion – lingual lipase, salivary amylase
 Taste – moistening, dissolves food so taste buds get full experience
 Antimicrobial – physical removal, WVC, opsonins, dimeric IgA, lactoferrins
 Protections – reduce temperature, buffering, protects stomach

Question 13

Unique Properties of Oral Secretions

Answer 13

Large volume relative to gland size; low osmolality, relatively high K+ concentration compared to plasma

Question 14

Esophageal Secretions

Answer 14

o Minor serous and mucous glands; primarily in lower segment that helps lubricate the bolus

Question 15

Gastric Secretions

Answer 15

 HCL – chemical digestion; secreted by parietal cells
 Intrinsic factor – required for B12 absorption by protecting from pancreatic proteases; secreted by parietal cells
• R protein – protects vitamin B12 from degredation; high affinity in acidic environment; cleaved by trypsin in duodenum and intrinsic factor takes over
 Pepsinogen – cleaved to pepsins by acidic environment that digest proteins; secreted by chief cells
 Gastrin – stimulates gastric motility and HCL protection in response to stretch by food; secreted by G cells in antrum of stomach
 Gastric lipase – lipid digestion; secreted by chief cells
 Other -
• Somatostatin – inhibits HCl secretion
• Histamine – stimulates HCl secretion
• Ghrelin – hormone secreted into blood when fasting and acts on hypothalamus to stimulate hunger; opposes satiety effects of leptin, Peptide YY, and GLP-1
 Mucus – lubricates/protects gastric mucosa from low pH

Question 16

Gastric Secretory Cell Types and their Products

Answer 16

 Surface mucous cells – protection, replace cells in lumen after desquamation
 Mucous neck – protection, progenitors of other cells lower in pit
 Parietal cells – HCl and intrinsic factor
 Chief Cells – pepsinogens, gastric lipase
 Endocrine cells – somatostatin, gastrin

Question 17

Regulation of Gastric Secretions

Answer 17

 Endocrine – gastrin and GIP act directly on parietal cells; secretin and peptide YY act indirectly by suppressing gastrin
 Paracrine – enterochromaffin-like cells (ECL) secrete histamine and serotonin
• Histamine increases HCl secretion locally by acting on gastric pits
• Serotonin is a vasoactive neuropeptide that causes constriction in intestines associated with peristalsis
 Neural – vagal Ach act on Somatostatin D cells, parietal, and mast cells; can stimulate GRP also

Question 18

Gastric Secretion Stimulation/Inhibition

Answer 18

 Stimulation – parasympathetics (vagus), gastrin, histamine
• Parasympathetics/Ach – acts directly on parietal cell
• Gastrin – endocrine hormone acting on parietal cell
• Histamine – paracrine hormone acting on parietal cell
 Inhibition – secretin, somatostatin, GIP, peptide YY, prostaglandins
• Secretin – duodenal hormone that decreases gastrin  lowering HCl secretion
• Somatostatin – acts directly on parietal cell
• GIP (gastric inhibitory peptide) – endocrine hormone that decreases secretion of gastrin lowering HCL secretion
• Peptide YY – secreted from small intestine
• Prostaglandins – cytoprotective hormone

Question 19

HCl and Response to Meal

Answer 19

 Antibactericidal EXCEPT for Helicobacter pylori
 Response to Meal: HCl secretion increases, decreasing the gastric pH  chyme enters duodenum and secretin is secreted  decreases gastrin secretion in stomach and duodenum and stimulates pancreas to release electrolytes  increase in bicarbonate into small intestine increases pH, protecting the mucosa and allowing pancreatic enzymes to work efficiently

Question 20

Helicobacter Pylori and Chronic NSAID users

Answer 20

• secretes urease that produces ammonia and neutralizes the H+ around H. pylori allowing it to survive; however ammonia (NH4) gets converted to NH3 which is toxic to cells and causes inflammation
  • produces mucinase which disrupts the mucus/bicarbonate barrier and causes ulcers
  • Critical amount of H. pylori must be reached to cause ulcers
  • H. pylori responsible for 90% of non-chronic NSAID associated gastric and duodenal ulcers
  • Chronic NSAID use – prostaglandin inhibors which are cytoprotective to the gastric mucosa; therefore when taken on empty stomach acid can enter stomach in absence of food

Question 21

Phases of Gastric Secretions

Answer 21

 Cephalic (“anticipation”) – chemo and mechanoreceptors on tongue, nasal mucosa; vagal effects, gastrin, acid, enzymes
 Gastric (food in stomach) – local nervous secretory reflexes; vagal reflexes; gastrin stimulation
 Intestinal – acidic chyme enters duodenum; composition of chyme important; feedback to HCl secretion; decrease gastric emptying; nervous mechanisms & hormonal mechanisms

Question 22

Intestinal Secretions

Answer 22

-made up of intestinal mucosal secretions, pancreas, liver, and gall bladder secretions

Question 23

Intestinal Cells and Glands (paneth, goblet, endocrine, brunner)

Answer 23

 Cystic fibrosis transmembrane regulator – secrete ions and water
 Paneth cells – host defense, secrete zinc and lysozymes that attack bacteria
 Goblet cells – secrete mucus
 Endocrine cells – secrete gastrin, CCK, secretin, GIP, motilin, serotonin
 Brunner’s glands – located in first part of duodenum; secretes thick mucus and proteases; stimulated by vagus; inhibited by sympathetic
 Enterokinase is from cells in duodenum

Question 24

Intestinal Neural and Hormonal Regulation

Answer 24

 Chyme contact and distention will increase secretions via local and enteric reflexes
 Parasympathetics will increase secretions; sympathetic will decrease secretions
o Hormonal Regulation – composition of chyme stimulate different hormones

Question 25

Peptide/amino acid Duodenal Stimulus

Answer 25

Gastrin (G cells) | ↑ HCl (at parietal cells), ↑ antral motility, ↑↓ gastric emptying; ↑ intestinal motility

Question 26

Acid; long chain fatty acid Duodenal Stimulus

Answer 26

Secretin (S cells) | ↑ pancreatic, biliary, & intestinal electrolyte buffers (and enzyme) secretions; ↓ HCl by ↓ gastrin; ↓ gastric emptying

Question 27

Peptide and Fat Duodenal Stimulus

Answer 27

CCK (I cells) ↑ pancreatic enzyme secretions; ↑ gallbladder contraction, relax sphincter of Oddi (↑ motility in small and large intestines, increase mass movements in colon); ↓ gastric emptying

Question 28

Fatty acids/amino acids & glucose

Answer 28

GIP (K cells) | ↑ insulin secretion; ↓ gastric emptying; ↓ gastric motility (mixing)

Question 29

Ileum Secretions

Answer 29

secretes H+ and bicarbonate o Carbonic anhydrase in enterocytes of ileum and colon o H+ acidifies the lumen so bicarbonate is also secreted and acts as buffer in exchange for chloride

Question 30

Pancreatic Endocrine/Exocrine Secretions

Answer 30

o Endocrine – ductless, secreted into bloodstream  Insulin, glucagon, and somatostatin which can affect gastric secretion and motility, in addition to their other systemic effects (blood glucose) o Exocrine – out through ducts and into lumen  Enzyme secretions and electrolyte secretions  Pancreatic Acinar cells – capable of making both enzymes and buffers into duct • Final buffer product can be different from what intitially left the acinar cells to enter ducts depending on the rate of flow (similar to saliva concept)

Question 31

Other Pancreatic Secretions

Answer 31

o Pancreatic proteases are stored as zymogens and secreted as precursors; once in duodenum, enterokinase (secreted by duodenum) activates these enzymes  Trypsin (activated by enterokinase) activates all the other zymogens: chymotrypsin, elastase, carboxypeptidases A & B o Pancreatic amylase – digests carbohydrates; more active than salivary amylase o Pancreatic lipase, cholesterol esterase, and phospholipase digests fats  Pre co-lipase – activated by trypsin after entering duodenum; co-factor for pancreatic lipase o Trypsin inhibitor – in pancreas and duct; keeps trypsin from being activated in pancreatic and bile ducts; lose of inhibitor can cause pancreatitis

Question 32

Primary Stimuli for enzyme/electrolyte secretion by Pancreas

Answer 32

o Primary stimuli for enzyme secretions are CCK and insulin | o Primary stimuli for electrolyte secretions are secretin and vagovagal reflexes

Question 33

Colonic Secretions

Answer 33

o Mucus – goblet cells on surface and in crypts; stimulated by contact, parasympathetics, and enteric nervous systems o Bicarbonate in exchange for Cl- o Potassium in exchange for Na+ and water o Colon tends to “house” bacteria; MMC ends at distal ileum and the bacteria collects into colon which is okay because it is dryer and has less nutrients -feces contents - small amount of K+ and HCO3-

Question 34

Bulk Absorption Occurs

Answer 34

happens in small intestine; nearly 9L of fluid; large amount occurs with absorption of Na+

Question 35

Chronic Diarrhea

Answer 35

– lose a lot of bicarbonate (buffer) and therefore become acidotic

Question 36

Parietal Cells (details)

Answer 36

• Anatomical changes in stimulated parietal cell causes the secretion • Make H+ from metabolism of CO2 • HCO3 transport OUT of cell via HCO3/Cl antiport • Use H+/K+ ATPase to send H+ into lumen against a 10^6 gradient; K+ is taken into the cell and then leaks back into lumen o Number of pumps in membrane dictate amount of acid produced o Secretions/hormones & drugs (proton pump inhibitors) work by regulating number of pumps in membrane o Cell contains carbonic anhydrase that converts CO2+H20H2CO3  H2CO3 rapidly breaks down to H+ and HCO3- as long as H+ inside cell is kept low by transporting H+ ions out of cell  HCO3 released into blood in exchange of Cl-  Alkaline tide results as high amounts of HCO3- released into blood but luckily Cl- will travel back into lumen and some will bind to H+ o Canaliculi – increase surface area on lumen side to secrete substances